I think the best way would to be to create a textbox to capture user typing. Then you can act on the input with code like:
UserText = TextBox.Text
UserLen = Len(UserText)
compare expected length of input to UserLen and act accordingly
parse the string via mid(UserText,x) *note* my testing seemed to indicate 0 based index where to get the first character it was mid(string,0,1)
so to parse the text when you expect 3+4=7
if len(UserText)<>5 -> error
Arg1 = mid(UserText,0,1)
Arg2 = mid(UserText,2,1)
Result= mid(UserText,4,1)
Operand= mid(UserText,1,1)
SyntaxCheck = mid(UserText,3,1)
if SyntaxCheck <> "=" -> error
then have 4 checks for operand to find = "+-/*"
calculate result via Arg1 and Arg2 and compare to Result
hope this is clear and helps ;)